This invention relates to a method of forming a fine resist pattern possessing a sectional shape of a high aspect ratio, in the resist mask forming process in one of the steps of fabricating a semiconductor device.
In forming a resist pattern, a generally employed method is a so-called single-layer resist method comprising a step of applying and forming a single photoresist layer on a semiconductor substrate, and a step of exposing and developing by using an ultraviolet light. In this method, the resolution is low and the contrast is not high because the gradient of the exposed degree along the depthwise direction and the gradient of the film thickness reduction by a developing solution at the side wall of the unexposed part along the depthwise direction are overlapped. Furthermore, the sectional shape of the formed resist pattern may present such a shape that the side wall is an inclined surface with respect to the substrate surface. As a method of realizing a higher aspect ratio as compared with such method, a method of heating the resist film formed on the substrate, exposing in a pattern to near ultraviolet light, exposing the whole surface to far ultraviolet light, and removing the portion exposed to the near ultraviolet light is disclosed in Japanese Laid-open Patent No. 61-116838, Laid-opened June 4, 1986, and in the Collected Papers of the 47th Autumn Meeting of the Japan Society of Applied Physics, page 540, 30a-P-3, published Sept. 27, 1986.
According to the conventional resist pattern forming method combining the pattern exposure by near ultraviolet ray light and flood exposure by far ultraviolet light, although it is possible to realize a higher aspect ratio than in the conventional single-layer resist method, it was not fully satisfactory with a contrast value (gamma-value) of the resist, from the viewpoint of the ultra fine pattern and there was room for further improvement.